1. Field of the Invention
The invention relates to a vehicle, and more particularly to a vehicle that travels using electric power from at least one of an engine and a motor.
2. Description of Related Art
Recently, hybrid vehicles that travel using drive power from at least one of an engine and a motor are becoming more widespread. A hybrid vehicle may include a generator provided separately to the motor in order to generate electric power using the power of the engine.
Japanese Patent Application Publication No. 2007-196733 (JP-2007-196733 A) discloses a technique employed in a hybrid vehicle including an engine, a motor, and a generator for performing control (also referred to hereafter as “battery-less travel control”). The hybrid vehicle disconnects the battery from an electrical system including the motor and the generator and drives the motor using power that is generated by the generator using the power of the engine to travel the vehicle when an abnormality occurs in a battery that stores electric power for driving the motor.
Incidentally, a hybrid vehicle is installed with an inverter for driving the motor. Principal methods of controlling the inverter include a pulse width modulation (also referred to as “PWM” hereafter) control method and a rectangular wave voltage control (also referred to simply as “rectangular control” hereafter) method. In the rectangular control, a modulation percentage of a voltage conversion (a value corresponding to a ratio of an output voltage to an input voltage) is larger than that obtained in the PWM control, and therefore a motor output can be increased. However, a control accuracy (a control response) tends to be poor, leading to instability in an inverter output voltage. Therefore, the rectangular control is typically used only in a high vehicle speed region, while the PWM control is used under normal speed region which is lower than the high vehicle speed region.
During the battery-less travel control, meanwhile, the battery cannot be used as a power buffer, and therefore an accurate electric power balance must be achieved between the motor and the generator. When the vehicle speed reaches to a high vehicle speed region during the battery-less travel control, however, the inverter control method shifts from the PWM control to the rectangular control exhibiting poor control precision. Accordingly, the power balance may collapse when a required driving force varies rapidly, and as a result, a voltage (the inverter output voltage) applied to the motor may become unstable.